Psoriatic arthritis resembles rheumatoid arthritis but does not produce the antibodies characteristic of rheumatoid arthritis. Psoriatic arthritis occurs in about 30% of people with psoriasis (a skin condition causing flare-ups of red, scaly rashes and thickened, pitted nails). People with a certain gene (HLA-B27) and those who have affected family members are at increased risk of psoriatic arthritis. The cause of psoriatic arthritis is unknown.

Symptoms

Inflammation often affects joints closest to the tips of the fingers and toes, although other joints, including the hips, knees, and spine, are often affected as well. Often, the joints of the upper extremities are affected more. Back pain may be present.

The joints may become swollen and deformed when inflammation is chronic. Psoriatic arthritis affects joints asymmetrically (more on one side of the body than the other), more so than rheumatoid arthritis, and involves fewer joints.

The psoriasis rash may appear before or after arthritis develops. Sometimes the rash is not noticed because it is hidden in the scalp, navel, or creases of the skin such as between the back of the buttocks and thigh. The skin and joint symptoms sometimes appear and disappear together.

Diagnosis

A doctor's evaluation

Blood tests

X-rays

The diagnosis of psoriatic arthritis is made by identifying the characteristic joint inflammation in a person who has arthritis and psoriasis. Doctors also ask people whether they have a family history of psoriasis.

There are no tests to confirm the diagnosis, but blood tests for rheumatoid factor are done to exclude rheumatoid arthritis, and x-rays are done to show the extent of joint damage.

Prognosis

The prognosis for psoriatic arthritis is usually better than that for rheumatoid arthritis because fewer joints are affected. Nonetheless, the joints can be severely damaged.

Ustekinumab, given by injection, and apremilast, taken by mouth, can be used to treat moderate to severe psoriatic arthritis.

Some people take methoxsalen (psoralen) by mouth and undergo psoralen plus ultraviolet A light treatments (phototherapy). This combination relieves the skin symptoms and most of the joint inflammation but may not help inflammation of the spine.

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